Oxidation products of INCONEL alloys 600 and 690 in pressurized water reactor environments and their role in intergranul
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INTRODUCTION
INCONEL* alloy 600 used in pressurized water reactor *INCONEL is a trademark of Inco Alloys International, Huntington Woods, WV.
(PWR) environments is often found to undergo stress corrosion cracking (SCC). Under these conditions, hydrogenated steam at temperatures above 300 °C in combination with a stressed susceptible microstructure can lead to crack development. Several important results show the importance of grain boundaries in SCC of alloy 600 in PWR environments including the necessity of grain boundaries for the initiation of cracks,[1] the degree of carbide precipitation at grain boundaries affecting crack growth rates, penetration of oxides into the grain boundaries at the alloy surface,[2] and intergranular attack being found at grain boundaries that intersect the crack.[3] Airey states that ‘‘in-service degradation was exclusively intergranular in nature.’’[4] Thus, consideration of the behavior of the material will be limited to only intergranular SCC (IGSCC) even though transgranular and pseudo-intergranular cracking of the alloy have been reported. Research indicates that alloy 600 will not crack in pure steam or dry H2 environments.[1,7–10] Hence, any proposed cracking mechanism must account for the role of H2 in combination with steam. In particular, it has been reported that the highest SCC susceptibility occurs when the H2 partial pressures are near the Ni/NiO stability boundary.[10] Magnin et al.[1] studied crack initiation in single- and poly-crystalline samples of alloy 600 in PWR environJ.B. FERGUSON, Graduate Student, and HUGO F. LOPEZ, Professor and Chair of the Materials Engineering Department, are with the University of Wisconsin–Milwaukee, Milwaukee, WI 53202. Contact e-mail: jb@ blanknull.com Manuscript submitted October 15, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
ments. They found that cracks initiated and propagated in the poly-crystalline sample, but no cracks could be initiated or propagated in the single crystal unless it had been precracked by fatiguing. This suggests that without existing cracks, SCC cracks initiate at surface grain boundaries. The susceptibility of alloy 600 to IGSCC has led to costly repairs and development of other alloy alternatives such as INCONEL alloy 690, which contains roughly twice as much Cr as alloy 600 (15 wt pct in alloy 600 vs 30 wt pct in alloy 690). Alloy 690 is considered to be insusceptible to cracking. Mithieux et al.[5] tested Reverse U-Bend samples for over 10,000 hours and some for over 25,000 without finding cracking. However, another experiment in similar conditions showed that cracks could be propagated in precracked MWOL alloy 690 specimens in hydrogenated steam.[6] This suggests that the Cr content helps to inhibit crack initiation, yet it is not as effective in hindering crack propagation. II.
OXIDATION STUDIES
Since crack initiation is a surface phenomenon, alloy oxidation either in pure O2, pure H2O, or H2O/H2 mixtures provides key information on the active crack initiation mechanisms and has been investigat
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